The present invention relates to torsional vibration damping mechanisms, and more particularly, to such mechanisms of the type including viscous shear dampers.
Torsional vibration damping mechanisms have long been used to reduce the adverse effects of torsional vibrations or torque fluctuations in vehicle drive lines. Torsional vibrations, or torque fluctuations, hereinafter referred to as "torsionals", emanate primarily from the engine in the form of torque spikes, and occur primarily where there are abrupt changes in driveline torque, for example, upon rapid acceleration/deceleration or upon transmission ratio changes.
Most known prior art torsional vibration damping mechanisms have utilized springs, disposed in parallel with a mechanical friction device. Driveline torque is normally transmitted by the springs, and flexing of the springs attenuates or reduces the potential amplitude of the driveline torsionals. The mechanical friction device dampens or reduces the rate of spring recoil.
More recently, it has become known to utilize springs disposed in parallel with a viscous shear damper, in place of the mechanical friction device, to dampen the rate of spring recoil. Such a device is illustrated and described in U.S. Pat. No. 4,936,434, assigned to the assignee of the present invention and incorporated herein by reference.
In the device of the above-incorporated patent, the viscous shear damper includes a housing assembly comprising a pair of stamped sidewall members which define a fluid chamber, and a clutch member, which is typically also a stamped member. Each side of the clutch member cooperates with an adjacent sidewall member to define a viscous shear space.
A typical viscous shear damper of the type to which the present invention relates includes a radially inner reservoir portion, which also serves as an expansion chamber, to permit fluid to expand as the temperature increases during operation. Typically, the ID (inside diameter) of the reservoir is defined by a seal member which is fixed to the clutch member, and extends axially to engage each of the adjacent sidewall members.
It has been observed that when prior art torsion damping mechanisms are shut down after operation, some of the fluid flows into the reservoir of the viscous damper, thus creating voids in the damping cavity (i.e., between the clutch member and the adjacent sidewall surfaces). When the vehicle is operated again, the viscous fluid flows slowly, and more of the fluid tends to flow toward one side of the clutch member than the other. Such uneven flow of the viscous fluid can cause the clutch member to shift within the housing (i.e., closer to one sidewall member and further away from the other), which would cause the mechanism to lose some of its damping ability. The uneven flow of viscous fluid also has the effect of causing the filling of the viscous shear chamber to take even longer than it would normally take. Finally, the uneven filling of the viscous shear space can also cause an out-of-balance condition, which may be felt by the vehicle operator as roughness in the driveline, and which is especially undesirable in view of the fact that the whole purpose of the torsion damping mechanism is to remove roughness and torsionals.
The problem described above with the prior art torsion damping mechanism is especially likely to occur in those mechanisms in which the two reservoir portions (i.e., the portions of the reservoir forward and rearward of the clutch member) are of different volumes. The reservoirs being of different volumes is quite typical because of the limited space available, axially, adjacent the flywheel. In other words, the uneven flow of viscous fluid described above is even more likely to occur if there are different volumes of fluid in the forward and rearward portions of the reservoir.